In a long-distance telephone line via a submarine cable or via a communication satellite, the subscriber's line, in general, connected to both ends of the line is of a two-wire circuit and its long-distance transmission portion is of a four-wire circuit in which transmission and reception are independent of each other in order to amplify a signal or for some other purposes. In the mobile communications network using a mobile telephone (or cellular phone), the subscriber's line of an analog telephone is also of a two-wire circuit and its portion from a repeater station for receiving a signal of a mobile telephone to a switch, etc. is also of a four-wire circuit. In those cases, a connection region between the two-wire and the four-wire is provided with a hybrid circuit for performing a four-wire/two-wire conversion.
The hybrid circuit is designed to match with impedance of the two-wire circuit. However, since the impedance of the subscriber's line is irregular and versatile, it is difficult to obtain always a good matching condition. For this reason, a part of signal input into an input terminal of the four-wire side of the hybrid circuit tends to leak toward an output terminal of the four-wire side. As a result, a so-called echo is generated. Although the echo level is lower than the transmitting speech level, speech quality is greatly lowered and a speech hindrance occurs in the case where echo returns to the talker with a delay. In view of the afore-mentioned status, an echo canceler is developed as effective apparatus for preventing a generation of echo.
FIG. 1 shows a hardware block diagram of a two-wire/four-wire conversion circuit with an echo canceler. As shown in FIG. 1, the echo canceler 1 is located on a front stage of a hybrid circuit 2. The subscriber of an ordinary analog telephone is referred to as the "near-end talker" and the subscriber of a mobile telephone as the "far-end talker". A far-end voice signal input into the echo canceler 1 is represented by Rin; a far-end voice signal output from the echo canceler 1, by Rout; a near-end voice signal input into the echo canceler 1, by Sin; and a near-end voice signal output from the echo canceler 1; by Sout, respectively.
The echo canceler 1 comprises an echo path estimation circuit/echo replica generator 3, a control unit 4, an adder 5, and a non-linear processor 6. The echo path estimation circuit/echo replica generator 3 generates the same signal as an echo occurrable to the near-end voice signal Sin, using the far-end voice signal Rout. This signal is referred to as the "echo replica". This echo replica is subtracted from the near-end voice input Sin by the adder 5. The echo path estimation circuit/echo replica generator 3 makes a learning of cancellation of echo, using the output signal Rout to the hybrid circuit 2 and a residual echo which could not be canceled by the adder 5.
A digital signal is transmitted through the transmission line on the side of the far-end talker. A D/A conversion (generally, a .mu.-LAW conversion) is made between the echo canceler 1 for processing a digital signal and the hybrid circuit 2 for making a conversion to an analog line. For this reason, the relation between the far-end voice output Rout and the near-end voice input Sin is not linear. Therefore, a mere linear computation made by the echo path estimation/echo replica generator 3 is not good enough to fully and completely cancel the echo.
Thus, a non-linear processor 6 is employed in order to cancel the residual echo which remains uncanceled. In the case where only the far-end talker is engaged in speech, the near-end voice output Sout is constituted of an echo alone. Therefore, the non-linear processor 6 makes a switching such that transmission of the near-end voice output Sout is prevented or that the near-end voice output Sout is replaced by a pseudo noise. Those operations of the non-linear processor 6 are made under the control of the controller 4. That is, the control unit 4 detects a speechless condition of the far-end talker or detects a double talk condition. Also, the control unit 4 turns on/off the learning function of the echo path estimation, detects a single talk condition of the far-end talker and further controls a switching operation of the non-linear processor 6.
Incidentally, an echo path is conventionally estimated by detecting the far-end talker's voice appearing to Sin in the form of an echo. That is, in the case where the far-end talker's voice is faint or soundless, it is impossible to detect its echo path. Therefore, in the case where the far-end talker abruptly starts talking, it is difficult for the conventional echo canceler to make a response at a good timing. The result is that an echo is transmitted to the far-end talker.
Also, as mentioned above, when the far-end is in a speechless condition or when a double talk is present, it is necessary to stop the learning. However, a timing error of detection sometimes occurs when the above detection is actually carried out. Such an error of detection timing can be one of the causes for generating a noise by mis-learning. In order to minimize the effect of the error, it is necessary to set a learning constant such that a designated value is gradually varied. However, this results in nothing other than a slow-down of learning speed.
The slow-down of a learning speed gives rise to such a problem that an echo becomes even larger immediately after the characteristic of the communication line is varied. Because the echo path is greatly varied when, in particular, a new call is set, an echo occurrable to the far-end talker's side becomes larger until the learning of the echo canceler is converged.
A speech hindrance caused by echo becomes more significant as the propagation time of signal is increased. In particular, since a speech via the satellite is greatly delayed, echo can be a big problem. Also in the case of a mobile communication using a mobile telephone, a low bit rate coding is sometimes carried out for a communication between a base station and a mobile machine or for office-to-office communication, and a lot of exchange procedures are carried out at the repeater station. Therefore, since a significant delay occurs, the echo can be a big problem.